linux-kernel - Re: [PART4 Patch v2 1/2] numa: add CONFIG_MOVABLE

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Date:	Tue, 20 Nov 2012 14:25:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wen Congyang <wency@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Rob Landley <rob@...dley.net>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Jiang Liu <jiang.liu@...wei.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PART4 Patch v2 1/2] numa: add CONFIG_MOVABLE_NODE for
 movable-dedicated node

On Fri, 16 Nov 2012 19:58:09 +0800
Wen Congyang <wency@...fujitsu.com> wrote:

> From: Lai Jiangshan <laijs@...fujitsu.com>
> 
> All are prepared, we can actually introduce N_MEMORY.
> add CONFIG_MOVABLE_NODE make we can use it for movable-dedicated node

This description is far too short on details.

I grabbed this from the [0/n] email:

: We need a node which only contains movable memory.  This feature is very
: important for node hotplug.  If a node has normal/highmem, the memory may
: be used by the kernel and can't be offlined.  If the node only contains
: movable memory, we can offline the memory and the node.

which helps a bit, but it's still pretty thin.

Why is this option made configurable?  Why not enable it unconditionally?

Please send a patch which adds the Kconfig help text for
CONFIG_MOVABLE_NODE.  Let's make that text nice and detailed.

The name MOVABLE_NODE is not a good one.  It means "a node which is
movable", whereas the concept is actually "a node whcih contains only
movable memory".  I suppose we could change it to something like
CONFIG_MOVABLE_MEMORY_ONLY_NODE or similar.  But I suppose that
CONFIG_MOVABLE_NODE is good enough, as long as it is well-described in
associated comments or help text.  This is not the case at present.

> +#ifdef CONFIG_MOVABLE_NODE
> +	N_MEMORY,		/* The node has memory(regular, high, movable) */
> +#else

I think the comment should be "The node has only movable memory"?

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